Wednesday, October 20, 2010

One of the most intriguing predictions of Einstein's theory of gravity, General Relativity, is that light rays are bent by massive objects. For example, if two galaxies are closely lined up from our point of view, the foreground galaxy can magnify the image of the background galaxy or create multiple images, etc. The source for the gravity of the foreground object could even be the mysterious substance "Dark matter". Hence gravitational lensing is a tool that can tell us something about the distribution of Dark matter - see the preprint Gravitational Lensing

There are subtle issues with the compatability of the two workhorse theories of contemporary physics: special relativity and quantum mechanics. The EPR paradox from the 1930's (the E stands for Einstein) and Bell's Theorem from the 1960's are two of the classic examples. The weight of suspicion often tends to fall on the quantum side. Einstein, notably unenthusiastic about the fundamental role of probability in quantum mechanics, believed that "God does not throw dice". John Bell, when asked if quantum mechanics could be wrong replied, "I hesitated to think it might be wrong; but I knew it was rotten". Nonetheless quantum mechanics has turned out to be fantastically accurate and the basis for much our current understanding of matter and radiation - likewise for special relativity. The preprint Can quantum theory and special relativity peacefully coexist? discusses the issues.

Taken from my place looking west. I might have been shooting through a closed window, not sure. I didn't notice the "UFO" at the time and it didn't appear in the previous or the next frames, taken at the same place and direction with minutes. The crescent moon was also near the horizon just outside the frame to the left. What was it?

Monday, October 11, 2010

How do we know the distance to objects that are really, really far away, like planets, stars and galaxies? Astronomers use a cosmic distance ladder to correlate different kinds of evidence about distances. Mathematician Terence Tao explains the cosmic distance ladder in a set of slides with plenty of nice graphics.

Thursday, October 07, 2010

More and more objects are being found in the Kuiper Belt, out beyond the planet Neptune. Some of them are quite large - over 2,000 kms in diameter. Paired objects are also quite common, some of the pairs are only loosely bound by gravity. Some of these observation pose difficulties for the standard theories of Solar System formation. See Kuiper Belt may be born of collisions at Nature.

In this article we review the discovery of the accelerating universe using type Ia supernovae. We then outline ways in which dark energy - component that causes the acceleration - is phenomenologically described. We finally describe principal cosmological techniques to measure large-scale properties of dark energy.

Wednesday, October 06, 2010

Water vapor is actually significantly lighter than dry air. Hence a steam balloon or airship would have characteristics in between those of helium and hot air balloons. There are some issues, alas - condensation for example. The Flying Kettle web page describes one such project.

Monday, October 04, 2010

Cosmic rays, high energy particles, mostly protons and alpha rays, are bombarding our atmosphere. How are they accelerated and where do they come from? We don't know. It is thought that the cosmic rays originating within the Milky Way are generated by supernovas, though there are no direct observations. However, there are good reasons to believe that extremely high energy cosmic rays originate outside our galaxy. There's a review in the preprint The Search for the Sources of the Cosmic Rays One Century after their Discovery

The quantum tripwire can detect an intruder with a very low probability of being noticed and low probabilities of false negatives and false positives. It uses the quantum Zeno effect. It is reported to work in realistic conditions. Here's the preprint: An Invisible Quantum Tripwire.